The route is in fact called <front>. This is currently broken in 8.x, which you can verify by clicking the "Hide descriptions" link on admin/config. You will get a nice helpful error that the 'front' route does not exist. (See also attached screenshot.) That's why I'm re-opening this instead of opening a follow-up. Setting to needs review so the attached patch gets a date with the bot, but the fact that 8.x is currently passing tests is that we're missing test coverage for this.